The packaging and printing industries have used reel to reel process methods for applying printing inks onto long lengths of flexible polymer and paper material for many years. In this case the so called web of material is moved continuously and repeating ink patterns are applied to selected regions by various roller transfer processes such as flexography, offset printing and gravure printing. The transfer roller diameter sets the maximum repeat pitch of the pattern along the web flow direction. Recently, with the growing demand for low cost, flexible electronic devices, continuous reel to reel processes using flexographic and offset printing of inks with electronic functionality have been developed. These “on the fly” printing processes where the web is continuously moving and the unwind and rewind reels are continually operated are very fast and can apply ink patterns at web speeds up to many hundreds of meters per minute. There is, however, limited flexibility to change product material and design and minimum line width that can be reliably printed and have relatively poor registration accuracy to existing patterns on the substrate and consequently they are not suitable for the patterning of the most advanced multi-layer flexible microelectronic devices that are now required. In this case minimum linewidths of a few tens of microns and registration accuracies better than this are needed. Another disadvantage of these types of continuous printing processes is that because of the requirement to create a structured plate or roller to define the ink pattern they are only appropriate for very long process runs.
The microelectronic and medical industries also use reel to reel process methods to manufacture flexible electronic devices such as solar panels and sensors. In this case higher registration accuracy is often needed and the technique of screen printing is one method that is often used to achieve this. In this case a defined length of the flexible substrate is drawn from a drum and supported on a process chuck. A suitable masking screen is registered to the substrate and then an excess of ink which usually has some electronic functionality is administered and forced through the screen mask by the one dimensional motion of a blade over the length of the chuck to define the required pattern. Because of the requirement to contact the screen to the substrate both items are held stationary during the printing cycle while the blade moves across the screen. Hence screen printing is not a continuous process but operates on discrete substrate areas and the unwind and rewind reels are generally operated intermittently or web accumulator units are disposed each side of the screen printer to allow the rest of the production line and the unwind and rewind reels to operate continuously. Screen printing is limited to minimum line widths of about 100 μm so is unable to be used for advanced flexible micro-electronic devices where line widths down to tens of microns are required. In addition because of the requirement to create a screen to define the pattern, screen printing is more suitable for long repetitive process runs.
For short process runs the requirement to, create a screen or patterned roller or plate every time the pattern design is changed is a serious hindrance. This limitation has been overcome by the introduction of mask less printing processes based on drop on demand ink jet printing applied to both electronically functional and decorative inks. In this process a print head with a row or rows of nozzles is moved back and forth over the substrate in a two dimensional raster trajectory to cover a defined area. The firing of ink droplets from the nozzles is activated at appropriate times by electrical pulses so that the required pattern is defined in the deposited ink. This type of printing is often referred to as “Digital Printing”. It is much slower than the more classical printing methods described earlier but due to the droplet placement control can achieve much higher registration accuracy. Minimum linewidths below 50 μm are also achievable. Hence this type of printing has become the usual method for the printing of functional inks to make precision flexible micro-electronic devices.
When this type of ink jet printing process is applied to sheet substrates it is usual to place them on a chuck which moves the substrate in one axis while the ink jet head is set orthogonal to and moved in the other direction. Such “crossed axis” systems can be engineered to be fast, accurate and highly repeatable. When, however, ink jet printing is applied to continuous webs of flexible material it is usual to operate in a mode where the substrate is held stationary and the print head is moved in two dimensions over the full process area by means of a motorized double gantry system. Such arrangements lead to loss of speed, accuracy and repeatability compared to the “crossed axis” architecture.
All the printing methods discussed are additive pattern forming processes but for the case where the substrate has a thin film applied to the surface, subtractive processes involving pulsed laser ablation or laser exposure are often used for the formation of microelectronic devices. In these cases, to form the required pattern either pulsed lasers are used to selectively remove areas of or lines in the thin film coating by a the process of direct laser ablation or alternatively CW or quasi CW UV lasers are used to expose a resist layer with the required pattern which is then transferred into an underlying film by subsequent development and etching processes.
When these laser processes are used to pattern sheet substrates a variety of different methods are used to achieve the required relative motion between the laser and the substrate. For small substrates a stationary laser beam with the substrate moved in two axes on a chuck is most common. This is the most desirable arrangement in terms of delivering a beam from a stationary laser to the process head as the distance between the laser and the head is fixed. For large sheets, to minimize footprint, it is common to move the laser head in two axes over a stationary substrate. This is the least desirable arrangement in terms of delivering a beam from a stationary laser to the process head which in this case is moved over some distance in two axes as the path length can change significantly. A crossed axis approach, where the laser head moves in one axis and the substrate in the other, is generally the most common in terms of achieving the fastest head and substrate speeds and highest accuracy and repeatability and in having an acceptable optical solution to the one dimensional change to the path length between a stationary laser and a moving process head:
When, however, laser processing is applied to continuous webs of flexible material, because of the limitations caused by the associated unwind and rewind drums it is usual to operate in a mode where the substrate is held stationary and the process head is moved in two axes over the full process area by means of a motorized double gantry system. Such arrangements lead to loss of speed, accuracy and repeatability compared to the “crossed axis” architecture and suffer from beam delivery issues associated with the changing optical path lengths in two axes.
Hence in order to create the high resolution, accurately registered, patterns in the thin films required for the manufacture of flexible microelectronic devices by both additive and subtractive processes there is a requirement for apparatus that can move either ink jet heads or laser heads or both with respect to the surface of discrete lengths of accurately located sections of continuous web flexible substrates which are unwound from reels and rewound onto reels.